Piezoelectric crystal



y 1949- 'w. c. ELMORE ETAL 2,470,609 A I PIEZQELECTRIC CRYSTAL FiledDec. 10, 1945 INVENTOR. WILLIAM G. ELMORE JOHN F. MARSHALL ATTORNEYPatented May 17, 1949 ,=PIEZOELECTRIC .CRYSTAL r -William (irwillmore,Swarthmore; and-John F. ,Marshall- Primos, Pa., assignors to-the;United@States of. America asrepresented by the-Secretary of .War

i. Application December 10, 1945,-Serial No.- 634,083

8 Claims. (01. file-327) .fIhis-v invention relates to piezoelectriccrystals and-,more particularly. to. means for. .mounting piezoelectric.crystals.

Piezoelectric plates, usually cut from. natural crystals of quartz orturmaline,.find extensive use, for example in. frequency controlofoscillating. circuits as is well known in the radio art. There aremany'idifierent ways invwhich such crystal plates, generally termedcrystals, may be mounted for the purpose of utilizing the.piezoelectric. .effect. Among the"difficulties nor-- mally encounteredare. theoscillation damping efiec'ts inherent inthe usual means ofmounting the-crystal in aJholderJ; These damping effects may be used,therchoice depending primarily upon the desired frequencyof-operationand upon the .rdesired frequency-temperature: characteristics.Similarly, 'the crystal: may bemutilized in anydesi-redwmodeof:osci1lation. Under any such circumstances,--however, ra noda-l planecan be found, so that the crystal may be supported in the mannerhereinafter described. In accordance with the invention it istproposedto use strings orthreadsengaging the crystal at or near the nodal .planeto provide; a support for the. crystal. Theterm thread as a used herein.includes any filamentous or thread-.like-Jnaterial which'may or may notbe resilient.

areievidenced by a reducedcrystal Q, and gen- Referring. .iagainrtor-the .,-drawing,vtwoloops erally poorer, frequency stabilitycharacteristics. l3r-.and-- M of filamentous: material,;each loopTherefore'amongtheobjects ofthis invention slightly greater in lengththan. the perimeter are to provide a. means for mounting a. piezoof the'crystal in a .plane perpendicular to the electric crystal so as toimprove the operating Q Y axis,--areused. 1 .The loopsimay be engagedwith of the crystal, and to "reduce damping effects zo'thabar-crystalbyzslippingthemover the crystal, to which, the crystal maybesubjecteddurin for example fromoppositeiends thereof,- loop oscillations.l31beingadvanced r-romtheleft-andloop l4 being Other objects, advantagesand novel features advanced from-the right, .as-shown in the drawof theinventionwillrbecome apparent from-the ing, so that .they meet:andcontact the crystal description contained-hereinzwherein referenceat-substantially thenodalplane...Loops l3 and is made-to theaccompanying drawing.

A- crystal is *oftendesignated in accordance with itsorientationwith-respect to the crystallographicaxes of the naturalcrystalline structure. Thus, an X-cut quartz crystal used at relativelylow frequencies, for-example, may be in the form of arectangular barso-cut-iromthe natural crystal that its longitudinal axis-is orientedperpendicularly to an X (electric) axis. Forfuller description-ofcrystallographic axes of quartz-and various crystal'cuts-,=reiereneemaybe had to Radio Engineers' Handbook by Frederick E. Terman, page 488et-seqr, published by McGraw Hill Book Company, 1943.

Referring now to the drawing, there is shown, by way of example; an-X-cut bar crystal it, and at-orie sidethereof is drawna set of mutuallyperpendicular--crystal1ographic axes (X, Y. and Z), for convenience inillustrating its orientation. A fundamental mode ot'oscillation of sucha crystal is that in which mechanicat vibrations occur substantiallyalong the Y axispcrpendicw larly to-the X and Z axes as designated inthe drawing. Such a-mode of oscillation establishes anodal=planexsubstantially at the center of thelong-itudinal axis=ofcrystal iii.

Although an X-cut bar crystal of-- rectangular cross-section is rhereshown and referred to in detail,-it will be understood that a crystal ofany othershape or relative dimensions or type of cut I 4 are engaged bysuitable clasping. means, for example. byhooks .-(not-..shown) on byother threadsl3, I3", I and. I4", .respectively knotted thereto..Thusloop. I3 is engaged by 'knots l5 and is lying in the Y-Z plane tothreads I3 and; l3,.respective1y, and loop M by knots l1 and I8 lying inthe XY plane to threads'M', and M", respectively. Spring or yieldablemeansmay be utilized between the -mounting (not shown) and any or all ofthe threads 13-, 13,- M and I4 to apply tension to the threadsinthe-respective directions illustrated'. Alternatively, thethreads maybe elastic or include elastic portions to supply the desired tension.

The tensions applied at'knots i Sand It include components of -iorce inthe Y- direction; likewise, -the tensions applied at knots l'I-andi8'have components ofii'orce along the Y axis. However,

the components of force applied to knots l5 and I6 along the Y axis areopposite in direction to the components offoroe'resolved along thesameaxis. applied to -knots l1 and I8. Thereby, threads l3 and! are heldin engagement-with each other, and against the lateral edges and facesof crystal Ill. The loops and-supporting threads: areengaged andtensioned in -such manner. that threads. I3'-= and" I3" extendsubstantially along a Y?rZ planeand have components of forces.resolvedmainlyzzalong the Y and Z axes; threads M and 14" extendsubstantially along an X-Y plane and have components of forces resolvedmainly along the X and Y axes. The effect of applying tension at theseveknots of the loop and thread structure is clear if one assumesthreads [3, I3, I l and i i to be tensioned, for example by Springs, ina suitable framework or holder. It is apparent that bar crystal ill willassume a position of relative to the holder.

The nodal plane of a crystal may not, in some instances, besymmetrically located in the crystal plate. For example, the nodal planemay lie substantially centrally in the bar crystal I here shown, butinclined at a small angle to the Z axis. The window formed by thesupporting loops does not then coincide with the nodal plane, for theloops tend to form the window in a plane at a right angle to thelongitudinal axis. The operation of the crystal under such circumstancesis not greatly affected, however, for the restraint upon the crystal bythe thread support is slight. If it is desired to position the loopsaccurately at a nodal plane which is inclined with respect to a physicalaxis of the crystal, the loops may be held in place by applying somevarnish such as Glyptal, which has the further advantage of improvingthe breaking strength of many types of thread which may be used.Furthermore, if the threads are coated with Glyptal, engaged on thecrystal, and then baked at about 100 centigrade for a few hours, thethreads will be more firmly cemented to the sides of the crystal.Another manner in which the loops may be caused to retain engagementwith the crystal at a true nodal plane is by forming small nicks ordepressions in the edges of the crystal bar at nodal positions, so thatthe string supporting structure may be engaged in these depressions.

The engagement of the crystal at substantially a nodal plane, thelightness of the crystal thread supporting structure, and the smalltension forces required result in very low damping of the crystal andcorrespondingly improved Q, improved stability and improvedfrequency-temperature characteristics.

Any suitable framework or holder (not shown) may be utilized in whichthe supporting threads may be anchored to secure the crystal and stringarrangement here shown. The holder may be provided with conventionalelectrode elements supported near, but not touching, suitable faces ofthe crystal, and with external leads or connection terminals as isusual. Such crystal faces, for example faces l9 and which areperpendicular to the X axis in the illustrated embodiment, may, ifdesired, be plated with conductive material. The need for separateelectrode elements may be eliminated :by providing contact means betweenthe plated faces and the external connection terminals, as by lightlypositioning, against the plated surfaces, connecting wires or foil fineenough not to impose excessive restraint upon the crystal.

It will be apparent to those skilled in the art that many variations ofthe invention are possible. It is not desired to restrict the spirit orscope of the invention by the particular embodiment herein described.

What is claimed is:

1. In a holder for a piezoelectric crystal having a longitudinal axisand opposed pairs of rectangular faces substantially parallel thereto, asupporting structure including two loops extending about said crystalsubstantially in a plane perpendicular to said axis, four supportingthreads, means engaging one pair of said threads to one said loop atportions adjacent an opposed pair of crystal faces, means engaging theother pair of said threads to the other said loop at portions adjacentthe other opposed pair of crystal faces, and yieldable means supplyingtension forces in the supporting structure whereby said loops are drawninto engagement with each other and with said faces to maintain saidcrystal in substantially fixed position within said holder.

2. A structure for supporting within a holder an object having alongitudinal axis and faces substantially parallel thereto, includingtwo loops having resilient portions and extending about said solidsubstantially in a plane perpendicular to said axis, a first pair ofattaching means engaging one said loop substantially on opposed ends ofa first line in said plane, a second pair of attaching means engagingthe other said loop substantially on opposed ends of a second line insaid plane at right angles to said first line, said attaching meansconnecting said loops to said holder, whereby said loops are drawn bysaid resilient portions into engagement with each other and with saidobject to maintain said object in a substantially fixed position.

3. A structure for supporting an object, including two loops extendingabout said object substantially in a plane, a holder, a first pair oftensioning means mounted on said holder and secured to one said loopsubstantially on opposed ends of a first line in said plane, a secondpair of tensioning means mounted on said holder and secured to the othersaid loop substantially on opposed ends of a second line in said planeat right angles to said first line, whereby said loops are drawn intoengagement by said first and second pairs of tensioning means tomaintain said object in a substantially fixed position within saidholder.

i. A method of suspending a crystal in a holder to allow substantiallyundamped oscillations thereof which comprises placing two loops aroundsaid crystal perpendicular to its longitudinal axis and substantially atits nodal plane, connecting a pair of strands to one of said loops atpoints adjacent to opposite faces of said crystal, connecting anotherpair of strands at points on opposite faces adjacent to said previouslymentioned faces and applying tension to said strands.

5. A method of suspending a piezoelectric crystal to allow substantiallyundamped oscillation which comprises placing two flexible loops aboutsaid crystal perpendicular to its longitudinal axis, engaging one loopat points adjacent to opposing faces of said crystal by resilientstrands, engaging the other loop at points adjacent to a second pair ofopposing faces by resilent strands, and applyin tension forces to allsaid strands.

6. The method of forming a holder for a crystal which will allowsubstantially undamped oscillations which comprises placing two loops ofnon-rigid material about said crystal perpendicular to its longitudinalaxis and substantially at its nodal plane, attaching a pair of strandsto one of said loops at points adjacent to opposin faces of saidcrystal, attaching a second pair of strands to the second of said loopsat points adjacent to a second pair of opposing faces and applyingtension forces to all of said strands to cause said loops to engage andretain said crystal in the desired position.

'7. The method of forming a holder for a crystal having a longitudinalaxis and pairs of parallel opposed faces perpendicular thereto, whichcomprises placing about said crystal and perpendicular to saidlongitudinal axis, two loops having a pair of strands attached to eachat diagonally opposite points and applying tension forces to saidstrands to caus said loop to engage and support said crystal.

8. A method for suspending a vibrating object to allow substantiallyundamped oscillations thereof which comprise placin pliable threadlikeloops of material about said object perpendicular to its axis ofvibration and substantially at its nodal plane, applying tension to saidloops to cause said loops to engage whereby said object is tightlyencircled at said nodal plane, applying Glyptal-like substance to saidsupporting REFERENCES CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Name Date 1,228,813 Osley June 5, 19171,567,225 Barbin Dec. 29, 1925 1,692,063 Trogner Nov. 20, 1928 1,875,324Bower Sept. 6, 1932 1,907,986 Little May 9, 1933 2,255,495 Scholes etal. Sept. 9, 1941

